1. Field of the Invention
The present invention relates to techniques for readily producing steering devices having uniform energy absorption performance. Herein, ‘uniform’ means that variations (in energy absorption performance) from product to product are small.
2. Description of the Related Art
Known energy absorbing type steering devices include a cylindrical steering tube (often referred to as a column) that surrounds a steering shaft. The steering tube is formed by fitting an inner tube into an outer tube. If energy is applied to the steering shaft, the tubes become more deeply fitted in order to absorb the energy. Another known absorbing type steering device includes a steering shaft. The steering shaft is formed by fitting an inner shaft into an outer shaft. The shafts become more deeply fitted when energy is absorbed. In yet another known energy absorbing type steering device, a steering tube is attached to a vehicle body so as to be axially displaceable. In response to energy being applied to the steering shaft, the steering tube is displaced relative to the vehicle body in order to absorb the energy.
All the above-described energy absorbing type steering devices absorb energy applied to the steering shaft by allowing the inner shaft amber, which is fitted within the outer cylindrical member, to axially displace relative to the outer cylindrical member.
In a steering tube formed from an inner tube and an outer tube, the outer tube serves as an outer cylindrical member and the inner tube serves as an inner shaft member. In a steering shaft formed from an inner shaft and an outer shaft, the outer shaft serves as an outer cylindrical member and the inner shaft serves as an inner shaft member. In a structure in which a displaceable steering tube is attached to a vehicle body, an outer cylindrical member secured to a bracket that is affixed to the vehicle body serves as an outer cylindrical member. In this case, the steering tube serves as an inner shaft member.
During the manufacture of such energy absorbing type steering devices, variations in energy absorption performance from product to product within the same lot must be very small.
Japanese Unexamined Patent Publication No. 56-8755 and Japanese Unexamined utility Model Publication No. 56-6669 disclose techniques that address the above-described requirement. These references describe an energy absorption steering device, in which an inner shaft is pressed into an outer shaft and a fine member, such as a piano wire, is interleaved between the shafts, thereby minimizing variations in energy absorption performance from product to product.
In a steering shaft, the rigidity of outer and inner shafts should not be too great along the axial direction in order to provide the required energy absorption performance; however, the rigidity thereof should be sufficiently great in the rotating direction in order to prevent the shafts from rotating relative to each other. In order to satisfy these requirements, the above-described prior art steering devices ensure satisfactory transmission of torque by pressing an inner shaft having an oval cross section into an outer shaft having inner shape whose cross section is similar to the shape of the inner shaft. These prior art steering devices dispose a single piano wire between the shafts in order to prevent the axial rigidity from varying between products.
In the above-described prior art steering devices, the oval cross section of the shaft ensures the transmission of the required torque. However, the inner shaft and the outer shaft contact at least at one point in the cross section. Therefore, energy absorption performance is directly affected by differences in the finish of the outer surface of inner shaft and the inner surface of outer shaft and by the dimensional tolerances of these components. Thus, realization of uniform energy absorption performance is hindered.
A steering tube made of an inner tube and an outer tube also has the same problem. Differences in the finish of the outer surface of inner tube and the inner surface of outer tube, and the dimensional tolerances of the components, directly cause variations in energy absorption performance and impede uniform energy absorption performance.
In a structure in which a displaceable steering tube is attached to a vehicle body, direct contact of an outer cylindrical member that is fixed to the vehicle body via a bracket, and the steering tube results in large variations in energy absorption performance. In order to achieve uniform energy absorption performance, as shown in Japanese Unexamined Patent Publication No. 8-20341, a cylindrical spacer made of a rigid synthetic resin is interleaved between the steering tube and the outer cylindrical member. Thus, a special spacer must be provided to impart uniform performance.
Accordingly, it is an object of the present invention to realize techniques that significantly decrease the effect of differences in the tolerances of components and variations in the finishes thereof on energy absorbing performance as compared to prior art techniques, and more specifically, to realize techniques for setting the energy absorption performance of each product at a predetermined constant level, even if the tolerances of the components and variations in the finishes thereof differ between it products. It is another object of the present invention to provide such structures at a relatively low cost.